Fix: Ensure explicit undefined on optional members

[huntie]

Summary:
Motivation

See react/react-native#57628.

Flow optional object members permit undefined as a value, so the faithful TypeScript translation of {foo?: T} is {foo?: T | undefined}.

The bare {foo?: T} form is stricter under TypeScript compared to Flow:

// Flow's semantics

interface Props {
  onRefresh?: () => void;
}
const a: Props = { onRefresh: undefined }; // ✅ ok

// TypeScript with exactOptionalPropertyTypes: true (i.e. strict mode)

interface Props {
  onRefresh?: () => void;
}
const a: Props = { onRefresh: undefined }; // ❌ error

interface PropsFixed {
  onRefresh?: (() => void) | undefined;
}
const b: PropsFixed = { onRefresh: undefined }; // ✅ ok

This diff

This makes the translator emit the explicit | undefined on optional members by default. Optional members (?:) now translate as wider resulting types:

foo?: number => foo?: number | undefined
foo?: () => void => foo?: (() => void) | undefined

Members whose type already includes undefined (e.g. the maybe type foo?: ?T → null | undefined | T) are left unchanged (no duplicate | undefined). Required members are untouched.

NOTE: This is a behavior change: 3 existing fixtures are updated. This is borderline for counting as semver breaking for this project — emitted types are now wider.

Differential Revision: D113033807

[meta-codesync]

@huntie has exported this pull request. If you are a Meta employee, you can view the originating Diff in D113033807.